An electric motor (e.g., an induction motor), typically comprises a stator and a rotor core assembly that is rotatable in relation to the stator. The rotor core assembly may include a plurality of aligned laminated plates. The so-called squirrel cage of the rotor core assembly may include a plurality of rotor bars inserted through radial slots formed in plurality of aligned laminated plates. Rotor bars often are copper rotor bars, especially for medium-sized and large-sized motors. Copper's greater conductivity versus other materials, such as aluminum, enhances the electrical energy efficiency of motors. Accordingly, induction motors with copper rotor bars have higher breakdown torque. However, motors having such copper rotor bars have lower starting motor torque. Copper rotor bars are relatively expensive.
Loose rotor bars may sometimes be a source of rotor Failure. One conventional way to tighten the copper rotor bars is using than steel shims to line alongside of the rotor bars. These conventional thin steel shims are sized appropriately to ensure that the rotor bars are relatively tightly secured in the slot. Typically, these thin mild steel shims are about 0.01 inch to 0.015 inch. (0.25 mm to 0.38 mm) in thickness and function to tighten the rotor bars and to provide a barrier between the rotor bars and the laminated plates of the rotor assembly so as not to shave or abrade the soft copper rotor bars as they expand and shrink.
There still exists a need for improved electric induction motors having rotor bars that are not only sufficiently tightly held within the slot, but have high breakdown torque and also improved starting motor torque.